1. Field of the Invention
The present invention relates generally to subwavelength apertures for light transmission, and more particularly to a multi-ridged subwavelength aperture and a magnetic head having a media heating device including a coupled multi-ridged subwavelength aperture.
2. Description of the Prior Art
Subwavelength apertures are known for the transmission of optical energy, where the shape of the aperture significantly affects the properties of optical energy that passes through the apertures. Here, we refer to subwavelength apertures as a class of optical apertures which have dimensions generally less than that of the wavelength of light they transmit. One such well known subwavelength aperture is a C-shaped aperture in which optical energy that passes through it creates a single, approximately round hot spot in the near field away from the aperture. Such a subwavelength aperture can have applications in hard disk drive devices that employ thermally assisted recording of magnetic data bits onto magnetic hard disk media.
Hard disk drives generally include one or more rotatable data storage disks having a magnetic data storage layer formed thereon. Data in the form of small magnetized areas, termed magnetic data bits, are written onto the magnetic layers of the disks by a magnetic head that includes magnetic poles through which magnetic flux is caused to flow. Magnetic flux flowing from a pole tip portion of the magnetic poles in close proximity to the magnetic layer on the disk causes the formation of the magnetic bits within the magnetic layer.
The continual quest for higher data recording densities of the magnetic media demands smaller magnetic data bit cells, in which the volume of recording material (grains) in the cells is decreased and/or the coercivity (Hc) is increased. When the bit cell size is sufficiently reduced, the problem of the superparamagnetic limit will provide a physical limit of the magnetic recording areal density. Present methods to delay the onset of this limit in storage media include the use of higher magnetic moment materials, and using thermally assisted recording (TAR) heads. An exemplary embodiment of the present invention relates to such thermally assisted recording heads in which a heating device is disposed within the magnetic head. Heat from the heating device temporarily reduces the localized coercivity of the magnetic media, such that the magnetic head is able to record data bits within the media. Once the disk returns to ambient temperature, the very high coercivity of the magnetic media provides the bit stability necessary for the recorded data disk.
In using optical energy for the heating of the magnetic medium, one needs to consider the applicability of the optics in near field, e.g., 1 to 20 nm from the source which resides in the magnetic head slider, and the heating of an area in the medium of very small dimensions, e.g., in the 20 to 30 nm range. Conventional diffraction limited optics is not applicable for heating such a small area. Such small spots can be produced in the near field of a subwavelength aperture. Significantly, the transmittance of a circular subwavelength aperture decreases as (r/λ)4 where r is the radius of the aperture and λ the wavelength of the optical waves. Thus the transmittance efficiency of a circular subwavelength aperture is very poor and high power lasers would be required to heat the medium. Recently, descriptions of several TAR methods for near-field heating of media have been published. In published U.S. patent application US2003/0184903 A1 and U.S. Pat. No. 6,944,101 special ridged waveguides are used as high transmission apertures disposed within the magnetic head and are taught for applications in perpendicular recording. These subwavelength apertures can produce a small spot in the near field with much higher through put than a simple circular aperture. In general the size of the heated spot depends on the optical wavelength and the dimensions and the composition of the materials for the waveguide/ridged waveguide.
Common to these prior art methods is that the heated spot is typically circular. In comparison therewith, a magnetic data bit of information in a magnetic medium typically has an elongated characteristic shape, such that its width (in the cross-track direction) can be a few times greater than the length of the data bit (in the down-track direction), and the aspect ratio, defined as (bit width)/(bit length), can be 3 or greater. Consequently, in TAR recording it will be beneficial to produce a heating spot that has approximately the same aspect ratio as the desired data bit.
An example of the present invention combines subwavelength apertures with an optical resonant cavity to amplify the intensity incident on the aperture and thus increase overall efficiency of transmitting light from the laser source to the medium. The coupling of power into the optical resonant cavity is by way of evanescent-wave coupling from an integrated waveguide. As a prior art example of this, R. W. Boyd et al., in Journal of Modern Optics, 2003, Vol. 50, No. 15-17, 2543-2550, “Nanofabrication of optical structures and devices for photonics and biophotonics” teaches a system consisting of a waveguide coupled to a resonant whispering gallery mode (WGM) cavity. In this technique a tapered planar waveguide is placed within a gap that is a fraction of a wavelength from a resonant microcavity.